This invention is directed to a method and apparatus for effecting precise alignment and bonding between surfaces, and in particular to methods and apparatus for aligning and bonding an integrated circuit wafer or chip and a patterned substrate.
Precise alignment of surfaces to be joined together has received continuing attention, particularly in the formation of electronic devices where, as in VLSI circuit elements, the patterning is microscopic or nearly microscopic. While the features and techniques of this invention are believed useful in any application that requires precisely aligning opposing surfaces, it will be described herein relative to the formation of integrated circuit devices, particularly VLSI devices, where the problem of positional alignment becomes more and more critical as circuit element, conductor, and connection point size is dramatically reduced.
Most particularly, in the manufacture of integrated circuits, it is necessary to have the capability to effect a precisely aligned bond between two parts. One may be an integrated circuit wafer or individual chip, which may have tiny solder, epoxy, or indium bumps formed thereon. The other may be a patterned substrate that comprises a pattern of tiny pads or conductors to be used as the electrical connections to the integrated circuit chip, or it may be the patterned surface of another semiconductor wafer, integrated circuit, or circuit part to be stacked. Herein, an upper part shall be referred to as a die and a lower part as a substrate. The bumps referred to are used to fuse the die to preselected points, such as conductive pads in the patterned substrate, after the two surfaces have been precisely aligned. Bonding is usually effected by known methods utilizing application of one or more of heat, pressure, or ultrasonic vibrations.
According to some prior methods or instruments for effecting alignment, two parts are positioned while they are spaced well apart, and thereafter, one is moved a considerable distance into contact with the other. Other instruments locate two elements to be joined in relation to reference marks, then move them together and join them. Such approaches introduce the likelihood, however, of misalignment of the surfaces when the areas to be joined are extremely small, e.g. measured in microns. The greater the distance a surface has to move, the greater is the alignment error that results from the smallest imprecision in the instrument's machined parts. An instrument available from the assignee of the current invention takes advantage of the transparency of certain semiconductors to infrared light by illuminating with infrared a die in place on a substrate to produce a video image. This works well when one part is transparent to infrared. But that part, the upper part, must have a polished surface to avoid a "frosted glass" type of diffusion that prevents visibility of the under surface through the overlying part. Another proposed apparatus looks down upon an upper part, and the operator, or electrical detectors, notes the correct alignment of its outline, not its actual connection points, with the underlying substrate. This only assumes that the points of contact are correctly positioned for bonding, and like many of the currently known pieces of equipment, it gives no indication of parallelism between the surfaces. Since lack of parallelism, lack of alignment, or both can result in the absence of a required electrical connection or an incorrect connection, to determine that connections are as desired when the chip and substrate surfaces have been contacted, but not yet bonded, continuity checks have been employed in which current has been supplied via connectors of the device being assembled and outputs have been measured. This is time-consuming and requires an understanding of what the effect will be of applying current through the available connections to the parts being joined.
In the patent literature, there is proposed the production of a combined optical image of a beam lead integrated circuit device and a spaced substrate such as a PC board to which the device is to be attached. An optical imaging arrangement is introduced between the beam lead IC and the board. The optics direct images of the surfaces to a mirror located to be viewed by microscope. Alignment is checked by viewing the combined images through the microscope. However, there is not the ability to adjust the relative magnification, brightness or contrast of one image relative to the other to assure accuracy of the composite image. The device and substrate are relatively large, as are the areas to be joined. Continual microscope use to view alignment is less desirable than observance of a greatly enlarged image displayed on a screen and easily viewed.
Prior combined image producing instruments often do not have the ability to explore portions of the surfaces to be joined, but rather concentrate on the whole surface. Additionally, these instruments may fail to suggest a check for parallelism.